The invention relates to a high and low temperature resistant siloxane vacuum grease composition and its preparation method. The high and low temperature resistant siloxane vacuum grease composition is used for lubrication and sealing of high-low temperature vacuum systems and high-low temperature vacuum valves, and comprises the following components by weight: 84-93% of organosilicon polymer oil, 3-15% of silicon dioxide powder, 0.5-7% of a surface covering agent, and 0.05-2.0% of an additive. The high and low temperature resistant siloxane vacuum grease composition has excellent sealing performance and long-term storage stability, and at the same time has good compatibility with a contact surface, high and low temperature performance, adhesion performance, chemical inertness, and water resistance. The high and low temperature resistant siloxane vacuum grease composition can be used for lubrication and sealing of high vacuum systems, sealing of glass valves, ceramic ground joints, pistons, and faucets, as well as sealing elements of air pressure valves, vacuum equipment and pneumatic brakes, etc.